Assertion: ${}_{Z}{X^{A}}$ undergoes $2\alpha$-decays,$2\beta$-decays,and $2\gamma$-decays,and the daughter product is ${}_{Z-2}{X^{A-8}}$.
Reason: In $\alpha$-decays,the mass number decreases by $4$ and the atomic number decreases by $2$. In $\beta$-decays,the mass number remains unchanged,but the atomic number increases by $1$.

• A
  If both Assertion and Reason are correct and the Reason is a correct explanation of the Assertion.
• B
  If both Assertion and Reason are correct but Reason is not a correct explanation of the Assertion.
• C
  If the Assertion is correct but Reason is incorrect.
• D
  If both the Assertion and Reason are incorrect.